This invention relates to the domain of the treatment of industrial effluents.
More precisely, this invention relates to treatment of liquid effluents containing mainly metallic sulphates and that can also contain sulphuric acid (H2SO4).
This type of effluents must be treated to give pure effluents that can be disposed of into the natural environment. In the past, the treatment designed to purify this type of effluents containing metallic sulphates consisted of adding calcium hydroxide Ca(OH)2 to them to obtain a precipitate of calcium sulphate CaSO4, 2H2O, in other words gypsum.
However, this type of process has the major disadvantage that it also causes precipitation of metals originating from metallic sulphates in the form of metallic hydroxides Mea(OH)e.
There are two problems in using these compounds which are obtained in a mixture with gypsum.
Firstly, the content of polluting metals in the gypsum makes it very difficult to upgrade the gypsum, taking account of the very low imposed metal concentration thresholds at the entry to recycling systems.
There is also the problem of possible coloration by metallic gypsum hydroxides, which are white in the pure state, that compromises upgrades of gypsum for which the whiteness index is a key factor.
In other words, the gypsum obtained at the end of this process cannot be used in industry and must be put into a discharge. Consequently, the pollution will simply be displaced from the liquid effluent to a solid residue.